Parasites play an essential role in maintaining healthy ecosystems. However, as human activities continue to degrade the environment and contribute to climate change, it is essential to understand if and how parasite-host interactions has been affected. In this study, we examined the relationship between the bivalve species Chamelea gallina and trematodes. Trematodes are a group of parasites with no significant body fossil record. We analyzed the dynamics of the trematode population, including its diversity, prevalence, pit size, and pit aggregation, by studying 193 infested valves and 838 pits retrieved in fossil and dead assemblages of C. gallina from Adriatic Sea Basin (Italy). The samples were collected from Late Holocene marine succession (~3 ky cal B.P. with sub-centennial error) and thanatocoenosis from the modern seabed. Our analysis based on Gaussian finite mixture modeling shows no decrease in the number of trematode taxa infesting C. gallina over the past 3000 years. However, the prevalence of trematode infestation decreased significantly over time (one order of magnitude), along with the average number of pits, median size, and parasite pit aggregation. These changes suggest a severe decline (or collapse) of the trematode-C. gallina interaction in the Adriatic Sea during a time of increasing human influence on this land-locked basin.

Our study uses data from Holocene core samples and modern death assemblages to understand how human-induced environmental change in the northern Adriatic Sea (Italy) may have affected parasite-host dynamics in the economically important bivalve Chamelea gallina. Thirty-one radiocarbon dates confirm temporal distinctness between the periods before and after the onset of significant human influence and confirm that trematode prevalence has decreased by an order of magnitude over the past similar to 2 k.y. The median number of parasiteinduced pits per bivalve host and parasite aggregation has also decreased significantly, signaling a substantial decrease in the effective population size of digenean trematodes. Gaussian finite mixture modeling of pit size does not support the hypothesis of parasite extinction. Combined, these results indicate the (potentially ongoing) collapse of parasite-host interactions in C. gallina in concert with human influence on the Adriatic and its transition to an "urban sea."